Cyclic nucleotide phosphodiesterases (PDEs) constitute five major families of enzymes that hydrolyze cAMP and cGMP. They have different physicochemical, structural, and kinetic properties; different classes of drugs selectively inhibit their activities. One of these families, the cGMP-inhibited PDE )cGI-PDE) family, plays an important role in the physiological regulation of cardiovascular system (cardiac contractility and rhythm, vasodilation and platelet aggregation). Little is known about the distribution of cGI PDE in different regions of the heart and cardiovascular system, and the structural and functional relationships among these cGI PDEs. cGI PDE activity was found in the cytosolic and sarcoplasmic reticulum (SR) fractions from human, rat and canine cardiac tissues. On Western blots, anti-human platelet cGI PDE antibodies reacted with cGI PDE from human, canine, and rat cytosolic and SR cardiac fractions. Immunoprecipitation with anti-human platelet cGI PDE indicated that cGI PDE was a major enzyme in these fractions. After cytosolic and SR fractions were incubated with {32P}-gammaATP and cAMP-dependent protein kinase, two phosphorylated proteins (110 and 80 kDa) were immunoprecipitated from cytosol fractions, whereas three (-130, -110, and -80 kDa) were isolated from SR fractions. Whether this presumed proteolytic processing is of functional importance is not known. We have recently cloned the cDNA for a cardiac cGI PDE from a human heart cDNA library in lambda ZAP II (meacci et al., Proc. Natl. Acad. Sci. USA 89:3721, 1992). The cloned cDNA is being expressed in E. coli and baculovirus systems to characterize the structure/function relationships in wild type, mutants and various truncated forms of cardiac cGI PDE.